Thermodynamic Performance and Economic Feasibility of Booster Heat Pumps in Low-Temperature District Heating

The transition to renewable energy sources presents a challenge for many district heating networks. Traditionally run on fossil fuels, the systems are built for high forward temperatures. Since many renewable energy sources have a lower temperature, new district heating networks should be dimensioned accordingly. But what about the existing grids?

In South Korea, district heating is a very common way to heat domestic houses. One way to keep these systems and at the same time prepare them for a large-scale introduction of low-temperature energy sources is to introduce booster heat pumps into the systems. This would lead to better use of the lower forward temperature, and also reduce the need to use a peak boiler during the cold season.

For the end-user, a booster heat pump would lead to reduced use of district heating. But also an increase in use of electricity. If this is positive or negative from an economic perspective depends on the relative price levels of electricity and district heating, as well as on the heating demand. Looking into the economic aspects, a hidden challenge is revealed: who should pay for such an investment? Unless the district heating company also sells electricity, they have no incentive for such an introduction – apart from the improvement in environmental performance.

One way around this is if the district heating plant also produces electricity. In such a combined heat and power plant the efficiency of power generation can be increased when decreasing the forward temperature. Thus, an introduction of booster heat pumps could inspire district heating companies to produce both heat and electricity – and in a more efficient way. From a South Korean national perspective, that is clearly preferable over simply producing more heat.

The economic feasibility of the booster heat pump is strongly influenced by the efficiency of the heat pump system itself. If the price of electricity is high, the low efficiency of the booster heat pump reduces the economic feasibility. This indicates that further optimization studies are needed in order to increase the efficiency of the booster heat pump before they with certainty could become part of an affordable and clean district heating system.

Chulwoo Roh, Korea Institute of Energy Research (KIER), South Korea

This text is shortened by HPC.

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